This invention relates generally to cathode heating circuits and particularly to a circuit for directly heating a line cathode and for protecting the cathode from voltage and current surges.
U.S. Pat. No. Re. 30,195 to C. H. Anderson et al. discloses a guided beam flat display device having a diagonal dimension of approximately 127 cm. The horizontal dimension of the display device, therefore, is approximately 101.6 cm, and the vertical dimension is approximately 76.2 cm. The faceplate and backplate of the display device are supported against collapse due to atmospheric pressure by a plurality of vertically extending internal support walls which also divide the display device into a plurality of vertically extending channels. A screen, which luminesces when struck by electrons, is arranged on the internal surface of the faceplate. A visual display is generated by propagating electron beams in all of the channels and simultaneously deflecting the electron beams toward the phosphor screen. While traveling toward the screen, the electron beams are horizontally scanned across the channels so that each channel provides a portion of each total horizontal line. Details of flat panel display devices which operate in this manner are described in U.S. Pat. No. 4,117,368 to F. J. Marlowe et al. and U.S. Pat. No. 4,131,823 to T. L. Credelle. The electrons for all the channels are provided by a line cathode which extends across the entire horizontal dimension of the display device.
U.S. Pat. No. 4,167,690 to R. A. Gange describes a structure and operational method for a line cathode designed for use in flat panel display devices of the type described hereinabove. In the patented method, heating pulses having a 10 .mu.sec pulse width are applied to the cathode at uniformly spaced intervals of approximately 50 .mu.sec. Thus, the cathode is heated by the 10 .mu.sec heating pulses and cools during the 50 .mu.sec period between the pulses. Current is extracted from the cathode of propagation along the channels during the 50 microsecond cooling periods.
In the operation of the display device, the horizontal scan lines are sequentially generated at the appropriate vertical positions during the cooling period when current is extracted from the cathode. The vertical stepping on the screen and horizontal beam scanning are synchronized by the use of horizontal synchronization pulses. The heating pulses are applied to the cathode during the horizontal retrace time when no display is generated and thus the heating pulses also are synchronized with the horizontal synchronization pulses of the display device.
The method of operating a line cathode described in Gange Patent is satisfactory for the purposes intended. However, a line cathode is very sensitive and thus is subject to failure due to several potential problems. For example, if the horizontal synchronization pulses should cease, or if several sequential pulses should be missing, the voltage applied to the cathode should be substantially reduced. Also, when the display device is initially turned on, the voltage applied to the cathode should be very low, or zero, to prohibit voltage surges from damaging the cathode. Also, it is important for the operating voltage applied to the cathode to have risen to the full operating level before allowing any heating pulses to be applied to the cathode.
The present invention is directed to a circuit which provides heating pulses to a line cathode in synchronization with the horizontal sync pulses and which protects the cathode from failure occasioned by the above identified potential problems.